ORINDA -- Bring on the Caldecott Tunnel fourth bore. Bring on the traffic relief. And say goodbye forever to the frustrating daily change of direction in tunnel lanes at one of the Bay Area's most famous bottlenecks.

After four years of work, the $417 million fourth and probably final bore of the 76-year-old Caldecott Tunnel, will likely open this weekend, Caltrans reports.

But just how much relief will it bring drivers? It depends on which direction and when you drive.

The big winners are reverse commuters. Those driving east through the tunnel between Contra Costa and Alameda counties on weekday mornings will save an average of five minutes. The return westbound commute will be cut eight to 10 minutes per afternoon trip, traffic engineers report.

"That is a big savings when you consider it's an average," said Ivy Morrison, a spokeswoman for the fourth bore project.

Drivers traveling west on weekday mornings from Contra Costa toward Oakland and San Francisco, and eastbound in the afternoon -- will see little change in congestion or travel times.

That means most Contra Costa residents -- who kicked in $124 million in local sales taxes to jump-start the project -- will get the fewest weekday benefits.

But the new two-lane bore has other perks. It will give the Caldecott four freeway lanes in each direction all the time, ending the daily switch of directions of two lanes to match the dominant traffic flow -- westbound toward Oakland in the morning and eastbound toward Orinda in the afternoon.

"You're reducing congestion and providing a brand new wider tunnel with the most advanced safety features," said Randy Iwasaki, executive director of the Contra Costa Transportation Authority, a project partner with Caltrans. "The bottom line is this project improves the reliability of travel in this region."

The project is being completed on schedule by the end of the year, and just within the $420 million budget, Iwasaki said.

Although weekday commutes won't be improved much for Contra Costa drivers, weekends are another story. Engineers expect those drivers will save time and frustration on weekend tunnel trips.

Traffic volumes are now so unpredictable on Saturdays and Sundays that Caltrans crews switch traffic directions in the middle two tunnel lanes up to 13 times per weekend, triggering backups.

But after the fourth bore opens, drivers from Contra Costa will get four westbound lanes all the time, including those weekend trips to see UC Berkeley football games and sporting or cultural events in San Francisco.

"You never know now on weekends when the cone crews will change the lanes and you get stuck in traffic," said Amy Worth, chairwoman of the Metropolitan Transportation Commission and an Orinda councilwoman.

Worth said having four lanes in each direction eliminates the stressful weaving and merging that confronts motorists when the freeway narrows from four to two lanes before the tunnel. "People will know where to go."

The fourth bore also enhances safety because it comes with a 10-foot wide shoulder lacking in the other three bores, stronger fans to sweep out smoke and seven escape passages connected to the third bore.

Some motorists question why Caltrans and Contra Costa County couldn't build two new bores -- not just one -- to speed up traffic for the majority of commuters.

But Iwasaki said expanding the tunnel to 10 lanes wouldn't help because the freeway on each side of the tunnel is limited to eight lanes, and there is no room to widen it.

The approaching arrival of the fourth bore has stirred memories for people who attended the opening of the first two Caldecott bores in 1937 and the third bore in 1964.

"The third bore opening was a big event," said Tamara Aszklar of Clayton, who was 3 years old when she attended in 1964 and got a piece of the ceremonial ribbon. "The fourth bore will be a big deal in reducing the traffic mess at the tunnel," she said.

Leo Croce, a 90-year-old Walnut Creek resident, was a 14-year-old Boy Scout in 1937 when he volunteered to be a guide and speaker on one of the buses carrying people to the original tunnel opening ceremony. It was an exciting time, and changed the future, he said, noting that "the Caldecott Tunnel really opened up Contra Costa to a lot of growth and development."

Eastbound traffic flows through the middle bore of the Caldecott Tunnel in Oakland. (Dan Rosenstrauch/Bay Area News Group)

CALDECOTT TUNNEL FOURTH BORE
What: A two-lane tunnel will become the fourth bore of the Caldecott Tunnel on Highway 24 between Oakland and Orinda.
How it was built: Excavators dug holes from both sides, and crews reinforced walls with metal rods and concrete line as they went.
Soil disposal: Much of the dug-out soil was trucked to Treasure Island.
Financing: Money for the $417 million project included $194 million from the federal American Recovery and Reinvestment Act, $124.6 million from Contra Costa County sales tax for transportation, $50 million in Bay Area bridge toll money, $20 million in state congestion funds, $19 million in state highway funds and $11 million in federal highway funds.
Lead contractor: Tutor-Saliba Corp. of Sylmar
Fossil finds: During construction, paleontologists found fossils from a three-toed horse, a camel and an oreodont -- a now-extinct sheeplike mammal -- on the east side of the tunnel. On the west side, they found many fish scales and microscopic shell-bearing crustaceans, providing evidence that the area was once under water.
Details: Visit www.caldecott-tunnel.org.